Mark Stringer, an anatomist at the Otago School of Medical Sciences, responded.

Each bit of our ear does something different. The outer ear is a sound receiver,
the middle ear passes on vibrations caused by sound waves and the inner ear
changes these vibrations into nerve signals that our brain recognises as sound.
The inner ear also helps us with balance.

If you looked deep into your ear canal you would see the eardrum, a thin piece
of skin smaller than your little fingernail. On the inside of the eardrum is the middle
ear, a small air space housing the tiniest bones in our body. Sound waves reaching
our eardrum cause these tiny bones to pass on vibrations to a small coiled tube
called the cochlea, which is filled with fluid. The cochlea is in the inner ear, deep
within the hardest bone in our head where it is well protected. The cochlea signals
to our brain when it detects vibrations. The inner ear also has other tiny tubes filled
with liquid. This fluid moves when you move your head, sending signals to your brain
which tell you if you are moving, standing still, or laying down.

Frogs, whales, reptiles, birds and land animals all have ears, which is why they can
communicate with sound. But they don’t all have three parts to their ears. Fish
only have an inner ear to help them with balance although some can detect sound too.
Frogs and turtles have a middle ear with an eardrum on the outside of their head
behind their eyes. Most other animals including birds and whales have a short outer
ear canal. Land mammals have a sticking out ear made of cartilage. Some, like horses,
can move this outer ear to locate sound but although some of us can wiggle our ears,
we can’t move them this much!